Voltage detector using electro-optic material and interference of light beams

ABSTRACT

In a voltage detector using an electro-optic material whose refractive index is changed by a voltage developing in a selected area of an object under test, a light beam emitted from a light source is applied to a beam splitter, where it is split into a light beam advancing along a reference optical path and a light beam advancing along an optical path extended to an optical path length changing means made of the electro-optic material, and the light beams are returned from the reference optical path and the optical path length changing means after reflection to the beam splitter, where they are caused to interference with each other to provide an output light beam which is applied to a detector. The efficiency of light beam utilization is improved, and the voltage developing in the selected area of the object can be detected with high accuracy.

BACKGROUND OF THE INVENTION

The present invention relates to a voltage detector for detecting thevoltage developing in a selected area of an object to be measured suchas an electric circuit. In particular, the present invention relates toa voltage detector of the type that detects voltage by making use of thechange in light polarization that occurs in accordance with the voltagedeveloping in a selected area of an object to be measured.

Various voltage detectors have been used to detect the voltagedeveloping in a selected area of objects to be measured such as electriccircuits. Conventional voltage detectors are roughly divided into twotypes: in one type, the probe is brought into contact with a selectedarea of an object to be measured and the voltage developing in that areais detected; and in the other type, the probe does not make contact witha selected area of an object to be measured and instead an electron beamis launched into that area and the voltage developing in it is detected.

Voltage changes rapidly in fine-line portions of objects such asintegrated circuits that are small and complicated in structure, and astrong demand exists in the art for detecting such rapidly changingvoltage with high precision without affecting the condition of thefine-line portions. However, this need has not been fully met by theprior art voltage detectors. With devices of the type that detectsvoltage by bringing the probe into contact with a selected area of anobject to be measured, it is difficult to attain direct contact betweenthe probe and a fine-line portion of the object of interest such as anintegrated circuit. Even if this is successfully done, it has beendifficult to correctly analyze the operation of the integrated circuitsolely on the basis of the voltage information picked up by the probe. Afurther problem involved is that contact by the probe can cause a changein the operation of the integrated circuit. Voltage detectors of thetype that employs an electron beam has the advantage that they arecapable of voltage detection without bringing the probe into contactwith an object to be measured. However, the area to be measured withsuch voltage detectors has to be placed in vacuum and its surface mustbe exposed at that. In addition, the area to be measured is prone to bedamaged by the electron beam.

The prior art voltage detectors have a common problem in that they areunable to operate quickly enough to follow rapid changes in voltage andhence fail to achieve precise detection of voltages that change rapidlyas in integrated circuits.

With a view to solving these problems, it has been proposed by two ofthe present inventors (Japanese Patent Application No. 137317/1987 filedon May 30, 1987) that voltage be detected by making use of thepolarization of a light beam that changes with the voltage developing ina selected area of an object to be measured.

A voltage detector operating on this principle is schematically shown inFIG. 6. The detector generally indicated by 50 is composed of thefollowing components: an optical probe 52; a CW (Continuous-Wave) lightsource 53 typically in the form of a laser diode; an optical fiber 51for guiding a light beam from the CW light source 53 into an opticalprobe 52 through a condenser lens 60; an optical fiber 92 for guidingreference light from the optical probe 52 into a photoelectric converter55 through a collimator 90; an optical fiber 93 for guiding output lightfrom the optical probe 52 into a photoelectric converter 58 through acollimator 91; and a comparator circuit 61 for comparing the electricsignals form the photoelectric converters 55 and 58.

The optical probe 52 is equipped with an electro-optic material 62 suchas an optically uniaxial crystal of lithium tantalate (LiTaO₃). The tip63 of the electro-optic material 62 is worked into a frustoconicalshape. The optical probe 52 is surrounded with a conductive electrode 64and has at its tip 63 a coating of reflecting mirror 65 in the form of athin metal film or a multilayered dielectric film.

The optical probe 52 further includes the following components: acollimator 94; condenser lenses 95 and 96; a polarizer 54 forselectively extracting a light beam having a predetermined polarizedcomponent from the light beam passing through the collimator 94; and abeam splitter 56 that splits the extracted light beam from the polarizer54 into reference light and input light to be launched into theelectro-optic material 62 and which allows the output light emergingfrom the electro-optic material 62 to be directed into an analyzer 57.The reference light is passed through the condenser lens 95 and thencelaunched into the optical fiber 92, whereas the output light emergingfrom the electro-optic material 62 is passed through the condenser lens96 and thence launched into the optical fiber 93.

Voltage detection with the system shown in FIG. 3 starts with connectingthe conductive electrode 64 on the circumference of the optical probe 52to a predetermined potential, say, the ground potential. Then, the tip63 of the probe 52 is brought close to the object to be measured such asan integrated circuit (not shown), whereupon a change occurs in therefractive index of the tip 63 of the electro-optic material 62 in theprobe 52. Stated more specifically, the difference between refractiveindices for an ordinary ray and an extraordinary ray in a planeperpendicular to the light-traveling direction will change in theoptically uniaxial crystal.

The light beam issuing from the light source 53 passes through thecondenser lens 60 and is guided through the optical fiber 51 to bedirected into the collimator 94 in the optical probe 52. The light beamis polarized by the polarizer 54 and a predetermined polarized lighthaving intensity I is launched into the electro-optic material 62 in theoptical probe 52 through the beam splitter 56. Each of the referencelight and the input light, which are produced by passage through thebeam splitter 56, has an intensity of I/2. As already mentioned, therefractive index of the tip 63 of the electro-optic material 62 varieswith the voltage on the object being measured, so the input lightlaunched into the electro-optic material 62 will experience a change inthe state of its polarization at the tip 63 in accordance with thechange in the refractive index of the latter. The input light is thenreflected from the reflecting mirror 65 and makes a return trip throughthe electro-optic material 62, from which it emerges and travels back tothe beam splitter 56. If the length of the tip 63 of the electro-opticmaterial 62 is written as l, the state of polarization of input lightlaunched into that material will change in proportion to the differencebetween refractive indices for the ordinary ray and the extraordinaryray and to the length 2l as well. The output light sent back into thebeam splitter 56 is thence directed into the analyzer 57. The intensityof the output light entering the analyzer 57 has been decreased to I/4as a result of splitting with the beam splitter 56. If the analyzer 57is designed in such a way as to transmit only a light beam having apolarized component perpendicular to that extracted by the polarizer 54,the intensity of output light that is fed into the analyzer 57 afterexperiencing a change in the state of its polarization is changed fromI/4 to (I/4) sin² [(π/2)V/V₀ ] in the analyzer 57 before it is furtherfed into the photoelectric converter 58. In the formula expressing theintensity of output light emerging from the analyzer 57, V and V_(o) isa half-wave voltage. is the voltage developing in the object to bemeasured,

In the comparator circuit 61, the intensity of reference light producedfrom the photoelectric converter 55, or I/2, is compared with theintensity of output light produced from the other photoelectricconverter 58, or (I/4) sin² [(π/2)V/V₀ ].

The intensity of output light, or (I/4) sin² [(π/2)V/V₀ ], will varywith the change in the refractive index of the tip 63 of theelectro-optic material 62 that occurs as a result of the change involtage. Therefore, this intensity can be used as a basis for detectingthe voltage developing in a selected area of the object to be measured,say, an integrated circuit.

As described above, in using the voltage detector 50 shown in FIG. 6,the tip 63 of the optical probe 52 is brought close to the object to bemeasured and the resulting change in the refractive index of the tip 63of the electro-optic material 62 is used as a basis for detecting thevoltage developing in a selected area of the object of interest.Therefore, the voltage developing in fine-line portions of a small andcomplicated object such as an integrated circuit which are difficult tobe contacted by a probe or which cannot be contacted by the same withoutaffecting the voltage being measured can be effectively detected by thedetector 50 without bringing the optical probe 52 into contact with suchfine-line portions.

As was described above, with the voltage detector 50 shown in FIG. 6,the voltage developing in the selected area of the object is measured byutilizing the variation in polarization of the light beam in theelectro-optic material 62. Therefore, it is essential for the polarizer54 to extract only the light beam having the predetermined linearpolarization component from the light beam emitted from the light source53, and for the analyzer 57 to extract the predetermined linearpolarization component from the output light beam from the electro-opticmaterial 62. Accordingly, the efficiency of light beam utilization inthe foregoing voltage detector is not enough.

SUMMARY OF THE INVENTION

Accordingly, an object of this invention is to provide a voltagedetector which is simple in the arrangement of the optical system andhas high efficiency of light beam utilization, and which can detect withhigh accuracy a voltage developing in a selected area of an object undermeasurement.

The foregoing object and other objects of the invention have beenachieved by the provision of a voltage detector using an electro-opticmaterial whose refractive index is changed by the voltage developing inthe selected area of the object under measurement, which, according tothe invention, comprises: a light source for emitting a light beam;optical path length changing means made of the electro-optic material,for changing the effective optical path length of the light beamcorresponding to a refractive index of the electro-optic material;splitting means for splitting the light beam from the light source intoa light beam advancing along a reference optical path and a light beamadvancing along an optical path extended to the optical path lengthchanging means, and causing a reflected light beam from the referenceoptical path and a reflected light beam from the optical path lengthchanging means to interfere with each other to provide an outputinterference light beam; and guide means for applying the outputinterference light beam provided by the splitting means to detectionmeans.

In the voltage detector of the invention, the light beam from the lightsource is applied to the splitting means, where it is split into thelight beam advancing along the reference optical path and the light beamadvancing along the optical path extended to the optical path lengthchanging means made of the electro-optic material. The return lightbeams from the reference optical path and the optical path lengthchanging means are applied to the splitting means, where they are causedto interfere with each other to provide the output light beam. In thiscase, the effective optical path length of the light beam reflected fromthe optical path length changing means is changed, because therefractive index of the optical path length changing means is varied bythe voltage developing in the selected area of the object. If theeffective length of the reference optical path is constant, then theintensity of the output light beam changes with the voltage. Therefore,the voltage provided at the predetermined part of the object can bemeasured by detecting the intensity of the output interference lightbeam with the detection means.

The nature, principle and utility of the invention will become moreapparent from the following detailed description when read inconjunction with the accompanying drawings, in which like parts aredesignated by like reference numerals or characters.

BRIEF DESCRIPTION OF THE DRAWINGS

In the a drawings:

FIG. 1 is an explanatory diagram showing the arrangement of the firstembodiment of a voltage detector according to this invention;

FIG. 2 is a graphical representation indicating the dependence of outputlight intensity on a voltage for a description of the operation of thevoltage detector shown in FIG. 1;

FIG. 3 is an explanatory diagram showing the arrangement of the secondembodiment of the voltage detector according to the invention;

FIG. 4 is an explanatory diagram showing the arrangement of a part of avoltage detector according to the invention which employs a streakcamera as its detector;

FIG. 5 is an explanatory diagram outlining the arrangement of a streakcamera; and

FIG. 6 is an explanatory diagram showing the arrangement of a voltagedetector having been proposed in Japanese Patent Application No.137317/1987.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of this invention will be described with referenceto the accompanying drawings.

The first embodiment of a voltage detector of the invention, as shown inFIG. 1, comprises: an optical probe 10; an electro-optic material 12formed on the bottom wall of the optical probe 10; an electricallyconductive electrode 13 on a peripheral side wall of the electro-opticmaterial 12; and a reflection mirror 14 is formed on the end face of theelectro-optic material 12. The reflection mirror 14 is made of a metalor a multilayered dielectric film.

A beam splitter 16 is provided inside the optical probe 10. The beamsplitter 16 splits a light beam from a light source 53 into a light beamadvancing to a reflection mirror 11 and a light beam going to theelectro-optic material 12, causes a reflected light beam from theelectro-optic material 12 to interfere with a light beam reflected fromthe reflection mirror 11, and outputs the resultant light beam aftersplitting. The output light beam from the beam splitter 16 is appliedthrough a condenser lens 17 and an optical fiber 18 to a detector 19.

More specifically, in the voltage detector thus constructed, the lightbeam emitted by the light source 53 is applied through a condenser lens20, an optical fiber 21 and a collimator 22 to the beam splitter 16,where it is split into the light beam going to the reflection mirror 11and the light beam going to the electro-optic material 12. The lightbeam reflected from the reflection mirror 11, and the light beamreflected from the reflection mirror 14 after passing through theelectro-optic material 12 are returned to the beam splitter 16, wherethey are caused to interfere with each other to provide the output lightbeam, which is applied through the condenser lens 17 and the opticalfiber 18 to the detector 19.

The refractive index of the electro-optic material 12 is changed by avoltage developed in a selected area of an object under test, as aresult of which the effective optical path length in the electro-opticmaterial 12 is changed. If the thickness of the electro-optic material12 in the light-traveling direction is represented by l, and therefractive index of the electro-optic material 12 to which no voltage isapplied is represented by n₀, then the effective "go and return" opticalpath length is 2ln₀. If the refractive index of the electro-opticmaterial 12 to which a voltage is applied is represented by n, then theeffective "go and return" optical path length is 2ln. Therefore, thedifference between the effective "go and return" optical path lengthprovided when no voltage is applied to the electro-optic material 12 andthat provided when the voltage is applied to the electro-optic material12 (hereinafter referred to as "an optical path length difference", whenapplicable) is:

    2l|n-n.sub.0 |.                          (1)

As was described above, the effective optical path length of the lightbeam in the electro-optic material 12 is affected by the voltage appliedto the latter 12. Therefore, the light intensity I of the output lightbeam provided through the interference of the light beam reflected fromthe electro-optic material 12 and the light beam reflected from thereflection mirror 11 is changed by the voltage applied to theelectro-optic material 12 as shown in FIG. 2. That is, the output lightintensity I is:

    I∝cos.sup.2 [(π/2) (V/V.sub.0)+φ]            (2)

where V₀ is a half-wave voltage; V is the voltage to be measured, and φis a phase difference provided when no voltage is applied to theelectro-optic material; namely, an initial phase difference which isrepresented by the following equation (3):

    φ=2πc·ΔL/λ                    (3)

where c is the velocity of light; ΔL is a relative optical pathdifference, and λ is a wavelength. The relative optical path differenceΔL is:

    ΔL=2·|L-|(L'30 n.sub.0 ·l)|

where L is an optical distance between the beam splitter 16 and thereflection mirror 11, and L' is an optical distance between the beamsplitter 16 and the electro-optic material 12.

The initial phase difference φ may be controlled by a mechanism in whicha micrometer is provided to move the reflection mirror 11 thereby tochange the optical distance L between the reflection mirror 11 and thebeam splitter 16.

Thus, the light intensity of the light beam passed through the condenserlens 17 is detected in this manner, so that the voltage developing inthe selected area of the object under test is detected.

In the above-described voltage detector, the following method may beemployed: Another beam splitter (not shown) for extracting a referencelight beam from the light beam emitted from the light source 53 isprovided between the collimator 22 and the beam splitter 16, and theintensity of the reference light beam from the beam splitter is comparedwith that of the output light beam from the optical fiber 18 by acomparator circuit (not shown), so that the intensity of the outputlight beam is prevented from being affected by the fluctuation inintensity of the light beam emitted from the light source 53, so thatthe voltage can be detected more accurately.

As was described above, in the above-described embodiment of theinvention, the voltage developing in the selected area of the objectunder test is measured by utilizing the fact that the voltage affectsthe effective optical path length of the light beam in the electro-opticmaterial thereby to change the light intensity of the output light beamoutputted from the condenser lens 17 which has been subjected to opticalinterference.

Thus, in the above-described voltage detector, although the beamsplitter 16 is employed, as the predetermined linear polarizationcomponent is not extracted from the light beam, the efficiency of thelight beam utilization is greatly increased. Furthermore, as thedetector 19 only detects the light intensity, the light beam having beensubjected to the optical interference can be guided to the detector 19by means of only one optical fiber 18. Thus, the voltage detector of theinvention can detect voltages with high accuracy, being considerablysimple in construction.

The second embodiment of the voltage detector according to theinvention, as shown in FIG. 3, comprises an optical probe 23 including:the first electro-optic material 25 which is increased in refractiveindex when applied with voltage; the second electro-optic material 26which is decreased in refractive index when applied with voltage, thefirst and second electro-optic materials 25 and 26 being provided insidethe optical probe 23; a reflection mirror 27 made of a metal or amultilayered dielectric film, disposed on the outer surfaces of thefirst and second electro-optic materials 25 and 26; and a beam splitter28 arranged inside the optical probe 23. The beam splitter 28 acts tosplit an incident light beam into a light beam going to the firstelectro-optic material 25 and a light beam going to the secondelectro-optic material 26, and causes the return light beam from thefirst electro-optic material 25 and the return light beam from thesecond electro-optic material 26 to interfere with each other andoutputs the resultant output light beam after splitting. The light beamgoing to the second electro-optic material 26 from the beam splitter isapplied to the second electro-optic material 26 through a reflectionmirror 24.

In the voltage detector thus constructed, similarly as in the case ofthe first embodiment of the voltage detector shown in FIG. 1, thevoltage developing in a selected area of the object under measurementcan be detected by utilizing the fact that when the return light beamfrom the first electro-optic material 25 and the return light beam fromthe second electro-optic material 25 interfere with each other, thelight intensity of the resultant output light beam is affected by thevoltage applied to the first and second electro-optic materials 25 and26.

In the second embodiment of the voltage detector according to theinvention, the electro-optic materials 25 and 26 employed are oppositein refractive index change to each other when applied with voltage.Therefore, the change in the light intensity which is caused as a resultof the optical interference is greater than that in the first embodimentof the voltage detector; that is, the second embodiment is higher insensitivity than the first embodiment.

In the first and second embodiments of the voltage detector, parts ofthe reflected light beams which have returned to the beam splitter (16or 18) will advance to the light source 53 to adversely affect thestability of the latter 53. This difficulty can be eliminated by thefollowing method: An optical isolator (not shown) is interposed betweenthe collimator 22 and the beam splitter 16 (or 28) so that no light beamis returned to the collimator 22 or the light source 53 from the beamsplitter 16 (or 28). In addition, anti-reflection films may be coated onlight-incidence surfaces of the electro-optic materials 12, 25 and 26and on surfaces of the beam splitters 16 and 28.

In the above-described first and second embodiments of the voltagedetector, the electro-optic materials 12, 25 and 26 may be either of auniaxial crystal or an isotropic crystal.

The first and second embodiments of the voltage detector have beendescribed with the premise that, when the two return light beams afterreflection are caused to interfere with each other in the beam splitter16 or 28, traveling directions of the two return light beams in theinterference operation are completely coincided with each other. If, onthe other hand, the traveling directions of the two return light beamsare slightly shifted from each other, the output light beam provided asa result of the optical interference shows interference fringesspatially. When the relative optical path difference of the two returnlight beams is changed by the voltage applied to the electro-opticmaterial 12 or to the electro-optic materials 25 and 26, theinterference fringes moves so that the dark and light interferencepattern is inverted in brightness. Thus, in the case where the travelingdirections of the two return light beams are slightly shifted from eachother, similarly as in the case where the traveling directions of thetwo return light beams are completely coincided with each other, thevoltage developing in the selected area of the object under test can bemeasured by detecting the movement of the interference fringes, that is,the change in brightness of the dark and light interference pattern.

In the voltage detector of the invention, by using a CW light source asthe light source 53 and a fast response detector such as a streak cameraas the detector 19 and measuring high-speed voltage variation of theobject under test with high time resolution, the higher-speed variationof voltage can be detected with high accuracy.

FIG. 4 shows the arrangement of a part of a voltage detector accordingto the invention, which employs a streak camera 33. As shown in FIG. 4,the output light beam OUT provided as a result of the interference ofthe two reflected light beams in the beam splitter 16 (or 28) in theoptical probe 10 (or 23) is expanded by an expanding optical system 30such as a beam expander, so that parallel rays of light are madeincident on a light-receiving surface 31 provided on the side wall ofthe optical probe 10 (or 23) and applied to the streak camera 33 througha bundle of optical fibers 32.

The streak camera 33, as shown in FIG. 5, comprises: a slit 34 in whichends of the optical fibers of the bundle 32 are arranged in a line; alens 35 to which the rays of light passed through the optical fibers 32thus arranged are applied through the slit 34; a photocathode 36 towhich the rays of light focused by the lens 35 are applied in a line,thus providing electron beams arranged in a line; a pair of deflectionelectrodes 37 for deflecting the electron beams horizontally; amicrochannel plate 38 for multiplying the electron beams thus deflected;and a phosphor screen 39 to which the output electron beams of themicrochannel plate 38 are applied. In FIG. 5, the microchannel plate 38and the phosphor screen 39 are shown separated for convenience indescription; however, in practice, they are joined together forming oneunit. Furthermore, the lens 35 is shown cylindrical; however, generallyit is not cylindrical.

With the voltage detector having the streak camera, the observation maybe made with the traveling directions of the light beams in theinterference operation coincided with each other similarly as in theabove-described case, or it may be carried out as follows: The travelingdirections of the light beams are slightly shifted from each other, andthe interference fringes are expanded with the expanding optical system30, and a linear part of the resultant expanded interference fringes isextracted with the slit 34 of the streak camera for observation.

It is rather difficult to maintain the light-traveling directionscoincided with each other with high accuracy. However, if theinterference fringes are observed as a streak image FG with the streakcamera, then even if the light-traveling directions are slightly shiftedfrom each other the observation will not be adversely affected thereby;that is, the observation can be achieved with high accuracy being freefrom measurement errors which otherwise may be caused by the deviationin position of optical system for instance due to vibration.

In the above-described embodiment, the streak camera 33 is employed.However, the same effect may be obtained by the following method: Apulse light source such as a laser diode which emits light pulse withextremely short pulse width in synchronism with the voltage undermeasurement, may be employed as the light source 53, and a photoelectricconverter may be used as the detector, so that the high-speed voltagechange of the object is sampled with an extremely short period. Thelight pulse width emitted from the laser diode ranges from several psecto 100 psec. If a CPM dye laser is employed, the light pulse with thewidth of approximately 0.1 psec can be utilized.

It is preferable to paint an inner surface of the optical probe 10 or 23black except for the parts through which the light beams pass, therebyto prevent the scattering of light.

As was described above, in the voltage detector of the invention, thelight beam from the light source is split into the light beam advancingalong the reference light path and the light beam advancing along thelight path extended to the optical path length changing means, and thereturn light beams from these optical paths after reflection are causedto interfere with each other to provide the output light beam, which isapplied to the detection means for detecting the intensity thereof.Therefore, the utilization efficiency of the light beam emitted from thelight source is increased and the optical system can be simplified;especially the means for guiding the output light beam to the detectionmeans can be one optical fiber. Thus, the voltage in the selected areaof the object under test can be detected by the detector with highaccuracy.

What is claimed is:
 1. A voltage detector for detecting a voltagedeveloping in a selected area of an object to be measured, comprising:alight source for emitting a light beam; a first optical path includingfirst reflection means and a first electro-optic material for sensingsaid voltage developing in said object as a change in a refractive indexthereof; a second optical path including second reflection means;splitting means for splitting said light beam introduced from said lightsource into a first light beam going along said first optical path and asecond light beam going along said second optical path, causing returnfirst and second light beams reflected from said respective first andsecond reflection means to interfere with each other, and extracting anoutput interference light beam; and detection means for determining saidvoltage developing in said selected area of said object on the basis ofintensity of said received output interference light beam.
 2. A voltagedetector as claimed in claim 1, wherein said second reflection means ismovable in a direction parallel to said second optical path.
 3. Avoltage detector as claimed in claim 1, wherein said second optical pathfurther includes a second electro-optic material for sensing saidvoltage in said object as a change in a refractive index thereof, andsaid changes in said refractive indices of said first and secondelectro-optic materials are opposite to each other.
 4. A voltagedetector as claimed in claim 3, wherein said second optical path furtherincludes a reflection mirror disposed between said splitting means andsaid second electro-optic material.
 5. A voltage detector as claimed inclaim 1, wherein traveling directions of said return first and secondlight beams under interference operation in said splitting means areslightly shifted from each other, said splitting means extracts saidoutput light beam having spatial interference fringes, and saiddetection means detects a change in brightness of a dark and lightinterference pattern.
 6. A voltage detector as claimed in claim 1,whereinsaid light source is a CW light source; and said detection meansis a fast response detector; and said voltage detector furthercomprising: an expanding optical system for expanding said output lightbeam to parallel light beams; and guide means for introducing saidparallel light beams to said fast response detector.
 7. A voltagedetector as claimed in claim 6, wherein said fast response detector is astreak camera.
 8. A voltage detector as claimed in claim 6, whereinsaidguide means is a bundle of optical fibers; and said expanded outputlight beams are applied to a photocathode of said streak camera throughsaid bundle of optical fibers and a slit in which ends of said bundle ofoptical fibers are arranged in a line.
 9. A voltage detector as claimedin claim 6, wherein traveling directions of said return first and secondlight beams under interference operation in said splitting means areslightly shifted from each other, said splitting means extracts saidoutput light beam having spatial interference fringes, and said streakcamera detects a change in brightness of a dark and light interferencepattern.
 10. A voltage detector as claimed in claim 1, whereinsaid lightsource is a pulse light source for emitting a pulse light beam withextremely short pulse width in synchronism with said voltage in saidobject; and said detection means is a photoelectric converter.